1. Field of the Invention
The present invention relates to a noncontact charging device using a grid electrode, and to an image forming apparatus provided with the charging device.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copying machines, printers and facsimile units, images are formed as follows. By imparting electric charges to a surface of a photoreceptor on which surface a photosensitive layer is formed as an image nearing member and contains a photoconductive substance, the surface of the photoreceptor is uniformly charged. Subsequently, an electrostatic latent image corresponding to image information are formed in various image forming processes. The electrostatic latent image is developed by a toner-containing developer supplied from a developing section, so as to obtain a visible image which is then transferred onto a recording material such as paper. After that, the visible image is fixed on the recording material by heat and pressure given by a fixing roller so that an image is formed on a recording sheet. In the image forming apparatus as described above, a charging device is used for charging the surface of the photoreceptor. For example, the charging device includes: a charging wire (a discharging wire) that is an electrode for conducting corona discharge on the photoreceptor; a grid electrode which is an electrode disposed between the surface of the photoreceptor and the discharging electrode and to which appropriate voltage is applied, for controlling an amount of charges imparted from the charging wire to the surface of the photoreceptor, and thus carrying out control of charged potential of the surface of the photoreceptor; and a support member for supporting the charging wire and the grid electrode. As the grid electrode, for example, there are used a wire grid electrode made of stainless steel, tungsten or the like ingredient, and a porous plate-like grid electrode formed of a metal plate (grid substrate) of stainless steel or the like ingredient with a large number of perforations. Note that, upon manufacturing the porous plate-like grid electrode, etching or other methods can be adopted for making the perforations in the metal plate. The porous plate-like grid electrode manufactured by etching is referred to as an etching grid. Contaminants such as a toner are easily deposited on the wire grid electrode among the grid electrodes described above. Due to such deposition of the contaminants, the function for control of charged potential of the surface of the photoreceptor becomes insufficient, and this causes a problem to be solved that the potential of the charged surface of the photoreceptor is further uneven.
The charging device described in Japanese Unexamined Patent Publication JP-A 2006-113531 realizes a charging device that has high durability, generates no rust and the like, has the controllability of electrostatic charge potential that is hardly damaged even when some contaminants such as toner adhere to make it possible to stably control the electrostatic charge potential of a photoreceptor within a suitable range for a long period of time and is inexpensive, by forming a nickel plated layer including polytetrafluoroethylene particles on at least one surface of a substrate in a porous plate-like grid electrode.
The charging device described in JP-A 2007-256397 realizes a charging device that has high durability, generates no rust and the like, has the controllability of electrostatic charge potential that is hardly damaged even when some contaminants such as toner adhere to make it possible to stably control the electrostatic charge potential of a photoreceptor within a suitable range for a long period of time and is inexpensive, by forming a nickel layer including polytetrafluoroethylene on a part or the whole of the surface of a substrate and by setting the minor axis length of secondary agglomerates of polytetrafluoroethylene included in the nickel layer including polytetrafluoroethylene to be equal to or less than two times the thickness of the nickel layer including polytetrafluoroethylene in a porous plate-like grid electrode.